What size boiler needed

[sherylryan]

Hi there

Could someone please advise what size condensing boiler I would need to run the following all at once.

225m2 of underfloor heating
( calculates on length x width of rooms)

8 Rads at 1470 watts each
2 Rads at 1.176 watts each
4 Towel Rails at 200 each watts

DHW at 3000 watts

21m trench heating at 3500 watts

Thank you

 

[ShaunCorbs]

What’s the Heatloss of the ufh rooms ?

 

[sherylryan]

What’s the Heatloss of the ufh rooms ?

I will try and find the calcs, one room is all glass the other not so much. Bear with me while I look thank you

 

[sherylryan]

The heat loss on ufh rooms is
Manifold 1 - 308 watts
Manifold 2 - 386 watts

I hope I’ve read report correctly

 

[John.g]

Do you mean 3080 & 3860 watts?.

 

[sherylryan]

Do you mean 3080 & 3860 watts?.

I’m not sure to be honest it just shows this on my sheet. I’m picking it will be 3080 & 3860. that makes more sense. 🙂

I have also calculated both manifold heat output as per your advise last week and I get

41.34 LPM x 60 x Dt 12 divide 860 = 34 kw. So I’m guessing the boiler needs to be bigger than 35kw?

 

[ShaunCorbs]

I get 21kw providing your lpm is correct

Also ufh is a dt 7 not 12

 

[John.g]

I’m not sure to be honest it just shows this on my sheet. I’m picking it will be 3080 & 3860. that makes more sense. 🙂

I have also calculated both manifold heat output as per your advise last week and I get

41.34 LPM x 60 x Dt 12 divide 860 = 34 kw. So I’m guessing the boiler needs to be bigger than 35kw?

From "Central Heating Pump" thread, posts 58&59 show measured flow rate of 29.5LPM (5x3lpm+3x4lpm+1x2.5lpm) with 12C dT (52C-40C) which gives a (troubled) UFH output of 24.7kw, the manifold temperature of 52C seems very high for a TMV setting of 42/45C but maybe these readings were taken before reducing the boiler circ pump head.

Can you show where you got the 308 & 386 watts from or a full view of the heat loss page above. Probably best to leave the boiler at 26kw until you sort out the rad zoning and change the other manifold, you can then take another set of readings.

 

[sherylryan]

From "Central Heating Pump" thread, posts 58&59 show measured flow rate of 29.5LPM (5x3lpm+3x4lpm+1x2.5lpm) with 12C dT (52C-40C) which gives a (troubled) UFH output of 24.7kw, the manifold temperature of 52C seems very high for a TMV setting of 42/45C but maybe these readings were taken before reducing the boiler circ pump head.

Can you show where you got the 308 & 386 watts from or a full view of the heat loss page above. Probably best to leave the boiler at 26kw until you sort out the rad zoning and change the other manifold, you can then take another set of readings.

Those LPM on this thread is what is calculated on our spec sheet at time of building, I was just about to comment that that is not what our manifolds are doing, they are currently doing as per thread 58/59.

 

[John.g]

I wouldn't be too familiar with heat loss calcs but maybe the UFH 308 & 386 watts heat loss are per degC difference between ambient temp and required room temp so if you assume say 30C design dt then manifold 1 heat loss might be 9.2kw and manifold 2 11.6kw ??, dT may normally only be ~ 15C so only 4.6kw & 5.8kw required?.

 

[sherylryan]

I wouldn't be too familiar with heat loss calcs but maybe the UFH 308 & 386 watts heat loss are per degC difference between ambient temp and required room temp so if you assume say 30C design dt then manifold 1 heat loss might be 9.2kw and manifold 2 11.6kw ??, dT may normally only be ~ 15C so only 4.6kw & 5.8kw required?.

Yes it think you may be correct. I’m just trying to get my head around the whole system, at build we had a 24kw gas boiler to run Rads and trench heating and a 16kw heat pump to run both ufh manifolds later another 9kw heat pump was install, so troubled manifold was run of 16kw heat pump and manifold 2 off 9kw heat, all this was replaced with a 35kw boiler now downsized to 26kw. I don’t understand how you can removed 49kw output equipment and replace with 35kw,

Attached below is the Rads and trench heating we have installed, is the output of the Rads the heat required from boiler? Eg, total output of Rads is 15,242 watts so we require 15kw to heat these?

Hope this all makes sense John and sorry for all the questions.

 

[John.g]

Yes, if you have a rad output of 15kw then the boiler output must be 15kw, of course the room(s) demand may only be 25% of this once up to temperature. I have a 26kw 16 year old SE Firebird boiler jetted to 20kw but it only ever runs continuously for ~ 30 minutes in the morning to get the whole system up to temp, I run my heating 15/16 hrs/day and once house up to temperature then the boiler on cycle time is often only 30% say 6kw output.

Also, your UFH output seems very high, I often see numbers like outputs of 65/85W/M2, so your 225M2 should emit ~ 19kw, thats the total of the two manifolds if the UFH area is in fact 225M2 so how does one account for this?, you are getting 24.7kw out of one manifold.

 

[sherylryan]

Yes, if you have a rad output of 15kw then the boiler output must be 15kw, of course the room(s) demand may only be 25% of this once up to temperature. I have a 26kw 16 year old SE Firebird boiler jetted to 20kw but it only ever runs continuously for ~ 30 minutes in the morning to get the whole system up to temp, I run my heating 15/16 hrs/day and once house up to temperature then the boiler on cycle time is often only 30% say 6kw output.

Also, your UFH output seems very high, I often see numbers like outputs of 65/85W/M2, so your 225M2 should emit ~ 19kw, thats the total of the two manifolds if the UFH area is in fact 225M2 so how does one account for this?, you are getting 24.7kw out of one manifold.

I have attached two images one that shows the underfloor heating circuits and the second one shows the room in m2. Our boiler for the last 10 days has used 46 Litres of fuel a day, running both ufh manifolds, controlled by thermostats, the Rads have only run for 5 hours in the last 10 days

I hope you can read the numbers on the attached documents

 

[John.g]

You are using ~ 470kwh/day with a boiler output of ~ 400kwh/day at 85% efficiency.
Are you running the UFH 24/7 and what is the "average" ambient temp over the past 10 days.

 

[sherylryan]

You are using ~ 470kwh/day with a boiler output of ~ 400kwh/day at 85% efficiency.
Are you running the UFH 24/7 and what is the "average" ambient temp over the past 10 days

The average temp outside has been approx 11 degrees, inside 22.

I have attached two graphs that shows when the heating has been firing in the last 24 hours it has time for 7 hours 54 mins.

 

[John.g]

Ok thanks, so is the heating/boiler enabled for 24 hours each day? if it is then a boiler on (firing) time of 7.9hrs in a 24hr period is a 33% firing cycle or 7.7kw or 185kwh/day=18 litres/day, a long way from 46 Litres/day??.

 

[sherylryan]

Ok thanks, so is the heating/boiler enabled for 24 hours each day? if it is then a boiler on (firing) time of 7.9hrs in a 24hr period is a 33% firing cycle or 7.7kw or 185kwh/day=18 litres/day, a long way from 46 Litres/day??.

Sorry my apologies, we have a 460 litre tank. It has used half of this in 10 days ( 230 litres) yes boiler is enabled 24 hours each day.

 

[John.g]

18 LPD is probably more accurate than the tank estimated 23 LPD but one way or the other a average boiler input if even 10kw or 8.5kw output to keep your house warm is pretty good IMO, so what's the need to increase it to 34kw or whatever?

 

[sherylryan]

18 LPD is probably more accurate than the tank estimated 23 LPD but one way or the other a average boiler input if even 10kw or 8.5kw output to keep your house warm is pretty good IMO, so what's the need to increase it to 34kw or whatever?

When the boiler was orginally installed it was set to 35kw then downsized to 26 kw because the condensate trapped kept melting 6 times.

When DHW, 3kwh UFH manifolds, 22 kw? radiators 15 kw and tench heating 3.5kwh. The boiler struggles at 26kw

 

[John.g]

If you have a SS condensate trap installed now then no reason why boiler output cannot be increased.

 

[ShaunCorbs]

Common issue with oil condy traps most have been converted from plastic to stainless from the manufacturer so might be worth a call

 

[sherylryan]

Common issue with oil condy traps most have been converted from plastic to stainless from the manufacturer so might be worth a call

We had a stainless one installed from the start and pipe still melted. I’m lost as to why this boiler is not performing as it should.

When they down sized the boiler to 26 kw they replaced the burner to a Riello RDB 2.2 26kw - 41Jx with a 26 kw rated nozzle.

the orginal burner was a Riello RDB 2.2R 33kw - 54 kw burner rated to 35 kw with the nozzle. I thought this burner was big considering we were in the lowest possible output of it..

 

[ShaunCorbs]

The outlet pipe melted what temp is it running at should be set at 60 c

 

[John.g]

If UFH is the main form of heating this can lead to very low boiler return temperatures and high condensing rates which just may lead to excess sooting if combustion settings are not spot on as per FGA. This "melting" problem was very well known here but seemed unique to the downward firing Firebird, it also be interesting to see any flue gas temperatures and also I think the temperature can be measured after the primary heat exchanger.

 

[sherylryan]

The outlet pipe melted what temp is it running at should be set at 60 c

Yes boiler is set to 60 c. Melted pipe and damaged baffles